1. Field of the Invention
The present invention concerns in general terms a method for the dynamic control of the inclination of an antenna for a radiotelecommunication system, notably for a cellular mobile radiotelephony system.
2. Description of Related Art
A radiotelecommunication system of the cellular mobile radiotelephony type has, as is well known, a plurality of fixed base stations communicating with a plurality of mobile units. Each of the base stations, defining a cell, covers a geographical area in which the mobile units can communicate with this base station. This geographical area is referred to as a sector.
It is well known that the sector is determined by the antenna parameters of the base station and notably by the position parameters of this antenna with respect to its immediate environment. Naturally, the height of the antenna with respect to the ground is an important parameter in determining the sector. However, another important parameter is the inclination of the antenna in a vertical plane. Normally, the antenna extends in a horizontal direction, that is to say the axis of the antenna beam is horizontal. The antenna can however be inclined in a vertical plane, that is to say the axis of the main antenna beam can be inclined by a certain angle upwards or downwards.
It is well known, as described, for example, in international patent application WO 97/29557 or in international patent application WO 99/17576, that inclining the antenna downwards notably reduces the problems of interference between cells using identical or adjacent frequencies. This is because the signals emitted by a downward-inclined inclined antenna are weaker at a great distance from the antenna than when they are emitted by a horizontal antenna, whilst they remain identical or even more intense close to the antenna. FIG. 1 shows an example of the impact of the inclination of the antenna downwards on the coverage of the cell. An antenna 10 whose lobe is directed horizontally emits a main beam 12. The result is a coverage area or sector 11. An antenna 20 which is identical but whose lobe is inclined downwards in a vertical plane by an angle of inclination xcex8 emits a main beam 22. The result is a coverage area or sector 21. As can be seen in FIG. 1, the sector 21 is reduced, with respect to the sector 11, in the direction of the axis of the main beam where the interference with adjacent cells is the greatest. On the other hand, the coverage of the area served by the antenna remains assured and is even increased. The interference produced by one cell on adjacent cells can therefore be thus advantageously reduced.
By inclining the antenna upwards, it is notably possible to reduce the screen phenomena occasioned notably by buildings situated close to the antenna. By inclining the antenna, and therefore the axis of the main beam, upwards, it can be ensured that a major part of the beam avoids the building. Because of electromagnetic diffraction, the coverage area can then extend behind the building. FIG. 2 shows an example of the impact of inclining the antenna upwards on the coverage of the cell. An antenna 30 whose lobe is directed horizontally emits a main beam 32. This beam strikes a building 33, which cuts it. This results in a coverage area or sector 31 limited to the area situated between the antenna 30 and the building 33. An antenna 40 which is identical but whose lobe is inclined upwards in a vertical plane by an angle of inclination xcex8 emits a main beam 42 which for the major part passes over a building 43 identical to the building 33. The electromagnetic diffraction means that an area situated behind the building is covered. The result is a coverage area or sector 41 consisting of an area 41A situated between the antenna 40 and the building 43 and an area 41B situated behind the building 43.
It is known that it is possible to determine, for each antenna in a communication system, an optimum inclination for obtaining optimum coverage. This determination can notably be achieved through a knowledge of the topology of the area served and a simulation of the electromagnetic propagation conditions.
However, in cellular mobile radiotelephony systems of the type based on code distribution multiple access (CDMA), for example, such an a priori determination of the optimum antenna inclination is not satisfactory. The CDMA technique is an example of a technique used for resolving the problems posed by the increasing number of users of cellular mobile radiotelephony systems. In this modulation technique, the frequency spectrum can be reused many times, which increases the capacity of the system. Determining the coverage necessary, in the context of such a technique, is difficult. According to the capacity of the cell, that is to say the number of users present in the cell, the coverage can be greater or lesser.
When the cell is highly loaded, that is to say when there are a large number of users present in the cell, the transmission power of the base station must be shared between a large number of signals. Consequently the coverage is reduced. This phenomenon is well known in the art of cellular mobile radiotelephony systems using the technique of code distribution multiple access (CDMA) by the name of cell respiration. In addition, interference with adjacent cells is then a limiting factor in relation to the total capacity of the system.
Generally, a telecommunications system offers uniformity of traffic neither in space nor in time. The compromise to be achieved between coverage and capacity within a cell seems to be very difficult to achieve a priori.
The object of the present invention is notably to resolve the above problems.
To this end, it proposes a dynamic antenna inclination control method for a radiotelecommunication system, notably of the cellular mobile radiotelephony system type, the radiotelecommunication system including a plurality of fixed base stations communicating with a plurality of mobile units, each of the base stations defining a cell and having a transmission and reception antenna whose beam covers a sector in which the mobile units can communicate with the base station, the antenna being able to be inclined so as to modify the sector covered, the method including a step of priori determination of antenna inclination consisting of determining an inclination of the antenna which is optimum in relation to the sector covered, a step of dynamic estimation of cell load consisting of estimating a load on the cell at a given instant, an instantaneous inclination determination step consisting of determining, at the given instant, as a function of the optimum antenna inclination determined and the estimated load, an instantaneous inclination, and an antenna inclination control step consisting of controlling the inclination of the antenna as a function of the determined instantaneous inclination.
Thus it is possible to dynamically adjust the inclination of the antenna as a function of the load constraints estimated in real time. The inclination of the antenna is then regularly changed so as to establish the best compromise between coverage and capacity.
According to another aspect of the present invention, the inclination of the antenna tends towards the optimum inclination when the load is low.
When the cell is lightly loaded, that is to say when the number of users present in the cell is low, it is preferable for the coverage area to be at a maximum, so as to best cover the remote areas. The inclination used will then often be close to zero, that is to say the antenna will be directed substantially horizontally, except if it is necessary, for example, to avoid buildings, in which case it will be inclined upwards.
On the other hand, the more the load increases, the more the inclination of the antenna moves away from the optimum inclination and a horizontal inclination.
When the cell is more heavily loaded, the phenomenon of cell respiration means that the coverage must be reduced. Interference with adjacent cells also becomes a limiting factor in relation to the total capacity of the system. According to the invention, the coverage is then deliberately limited by increasing the inclination compared with the optimum inclination, that is to say by moving away still further from the horizontal, and then the surrounding cells must take over for coverage of the remote areas which are then less well covered. The decrease in coverage does not pose any problem because, the less interference the surrounding cells suffer from the cell under consideration, the better they are able to take over the coverage.
The method according to the invention is therefore better adapted to cases where the loads on adjacent cells are not uniform. The more unbalanced these loads, the more effective the method, which is the case, notably, when the geographical distribution of the base stations is not adapted to the geographical distribution of the traffic or when the geographical distribution of the traffic varies over time.
Thus according to another aspect of the present invention, the dynamic antenna inclination control method also comprises a step of dynamic estimation of load imbalance between cells consisting of estimating, at the given instant, the load imbalance between the cell and the adjacent cells, the instantaneous inclination determination step determining the instantaneous inclination also as a function of the estimated load imbalance.
When the load imbalance is low, the increase in the inclination of the antenna as a function of the load is lessened. In other words, when the adjacent cell loads are balanced, the antenna of one of these cells is moved away less from its optimum inclination when its load increases. This lessening can even result in there being no change in inclination when the loads on adjacent cells are balanced.
On the other hand, when the load imbalance increases, the increase in the inclination of the antenna as a function of the load is amplified. Advantageously, this amplification is proportional to a factor representing the load imbalance.
The load imbalance can be estimated by the exchange of information between the base stations, for example via a centralisation system.
According to another aspect of the present invention, the dynamic cell load estimation step estimates the load of the cell from the parameters peculiar to the base station.
The parameters peculiar to the base station can be a total transmission power level of the antenna, a total throughput of information transmitted by the antenna or a number of signals to be transmitted in the cell.